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United States Patent |
5,119,063
|
Nonnenmacher
,   et al.
|
June 2, 1992
|
Variable power resistor
Abstract
A variable power resistor includes a heat sink having a front face and a
back face with an electrically insulating, thermally conductive ceramic
coating bonded directly onto the front face such that the ceramic coating
is in direct thermal contact with the heat sink. A plurality of discrete
thick film conductive circuit pads are positioned on the electrically
insulating, thermally conductive ceramic coating and a thick film
resistive layer is positioned over portions of the conductive circuit pads
such that the pads are electrically connected in series. The variable
power resistor also includes a moveable contactor capable of contacting
the circuit pads in order to vary the resistance of the resistor and an
electrical connection between the resistor and an electrical circuit. The
electrically insulating, thermally conductive ceramic coating may be
plasma sprayed onto the heat sink, while the resistive circuit may be
screen printed onto the ceramic coating.
Inventors:
|
Nonnenmacher; Ronald C. (Apache Junction, AZ);
Schulz; Kathleen (Novi, MI);
Lewis; Richard C. (Merrimack, NH);
Riley; Richard (Riverside, CA)
|
Assignee:
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United Technologies Corporation (Hartford, CT)
|
Appl. No.:
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629885 |
Filed:
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December 19, 1990 |
Current U.S. Class: |
338/159; 338/51; 338/162; 338/164; 338/171; 338/172 |
Intern'l Class: |
H01C 010/10 |
Field of Search: |
338/160,159,162,164,171,172
252/514
|
References Cited
U.S. Patent Documents
3838071 | Sep., 1974 | Amin | 252/514.
|
4885434 | Dec., 1989 | Voltaggio et al. | 338/172.
|
5053283 | Oct., 1991 | Brown | 428/432.
|
Foreign Patent Documents |
0375163 | Jun., 1990 | EP.
| |
Primary Examiner: Lateef; Marvin M.
Attorney, Agent or Firm: Romanik; George J.
Claims
We claim:
1. A variable power resistor, comprising:
(a) a heat sink having a front face and a back face;
(b) an electrically insulating, thermally conductive ceramic coating bonded
directly onto the front face of the heat sink such that the ceramic
coating is in direct thermal contact with the heat sink;
(c) a plurality of discrete thick film conductive circuit pads positioned
on the electrically insulating, thermally conductive ceramic coating,
wherein the conductive circuit pads comprise about 10 wt % to about 70 wt
% of a lead borosilicate glass matrix, about 15 wt % to about 20 wt %
zirconium spinel reinforcing particles, and up to about 90 wt %
coprecipitated palladium and silver conductive particles in an amount
effective to provide a resistance of up to 0.5 ohms/square;
(d) a thick film resistance layer positioned over portions of the
conductive circuit pads such that the pads are electrically connected in
series through the thick film resistive layer, wherein the resistive layer
comprises about 5 wt % to about 70 wt % of a lead borosilicate glass
matrix, about 15 wt % to about 20 wt % zirconium spinel reinforcing
particles, and more than about 5 wt % coprecipitated palladium and silver
conductive particles in an amount effective to provide a resistance of
greater than 0.5 ohms/square;
(e) a moveable contactor capable of contacting the circuit pads in order to
vary the resistance of the resistor; and
(f) means for electrically connecting the resistor to an electrical
circuit.
2. The power resistor of claim 1, further comprising fins extending from
the back face of the heat sink.
3. The power resistor of claim 1 wherein the heat sink comprises a material
selected from the group consisting of aluminum, copper, and zinc.
4. The power resistor of claim 1 wherein the electrically insulating,
thermally conductive ceramic coating comprises alumina.
5. The power resistor of claim 1 wherein the electrically insulating,
thermally conductive ceramic coating is about 0.002 in to about 0.010 in
thick.
6. The power resistor of claim 1 wherein the conductive circuit pads are
about 10 microns to about 40 microns thick.
7. The power resistor of claim 1 wherein the resistive layer is about 10
microns to about 40 microns thick.
Description
DESCRIPTION
1. Technical Field
The present invention relates to thick film electrical devices. In
particular, it relates to thick film variable power resistors.
2. Background Art
Power resistors are used in many applications to control electrical current
or voltage by dissipating electrical power. A resistor may have either a
fixed resistance or a variable resistance. Variable resistance power
resistors are useful in applications in which the ability to conveniently
adjust current flow or voltage is desirable. For example, variable
resistance power resistors are used as as dimmer controls in lighting
circuits, as components in automobile ignition circuits and test
equipment, and in other applications.
All power resistors generate heat when they dissipate electrical power. The
amount of heat generated is directly proportional to the amount of power
dissipated. The heat must be removed from the resistor to prevent it from
overheating and burning out. Heat removal is typically a function of the
resistor design, with the rate of heat removal directly proportional to
the thermal conductivity of materials used to construct the resistor and
the amount of resistor surface area exposed to a cooling fluid. The
cooling fluid is typically air.
Heat removal from a resistor can be enhanced by increasing the amount of
surface area available for heat transfer. This is often done by adding
fins to the resistor or by attaching the resistor to a finned heat sink.
However, depending on the design of the resistor, fins can interfere with
mechanical elements of the resistor. For example, fins could interfere
with the operation of a variable single wire power resistor, which has a
mechanical means for varying resistance. A conventional thick film power
resistor, however, has a design which is compatible with cooling fins.
A conventional thick film power resistor typically comprises a heat sink
attached to a ceramic plate, a thick film resistive circuit deposited or
printed on the ceramic plate, and a moveable contactor which facilitates
changing the resistance of the device. The heat sink, which may have fins,
is typically made from a metal such as aluminum. The ceramic plate serves
as an electrical insulator between the resistive circuit and the heat
sink. The ceramic plate may be attached to the heat sink with mechanical
means such as bolts or a spring clip or with a thermally conductive
adhesive. If mechanical means are used, a thermal grease must be used to
make thermal contact between the plate and the heat sink. If a thermally
conductive adhesive is used, the adhesive itself is sufficient to make
thermal contact between the plate and the heat sink.
Conventional thick film power resistors suffer from several drawbacks.
First, they can be somewhat cumbersome to assemble. The ceramic plate is
fragile and is subject to breakage if dropped during assembly. The plate
may also be damaged if mechanical means are used to attach it to the heat
sink. The need to use a thermally conductive grease or adhesive to make
thermal contact between the plate and heat sink adds an assembly step and
material cost. Second, although the ceramic plate is thermally conductive,
it contributes a significant thermal resistance between the resistive
circuit and the heat sink. The thermally conductive grease or adhesive
also contribute a significant thermal resistance. Finally, the thermal
grease tends to dry out over time, increasing its thermal resistance and
impairing the thermal contact between the ceramic plate and heat sink.
In some environments, for example gas turbine engines, ceramic coatings
have been plasma sprayed directly onto a base material to serve as a
thermal barrier. Thermally conductive greases and adhesives are not needed
with such coatings, and indeed, are incompatible with them. However,
because such plasma-sprayed ceramic coatings have been used as thermal
insulators, they have not been used in applications where an electrically
insulating, but thermally conductive material is required.
Accordingly, it would be desirable to have a thick film variable power
resistor which does not not have a breakable ceramic insulator and which
does not require a thermally conductive grease or adhesive to conduct heat
to a heat sink.
DISCLOSURE OF THE INVENTION
One aspect of the present invention includes a variable power resistor,
comprising a heat sink having a front face and a back face with an
electrically insulating, thermally conductive ceramic coating bonded
directly onto the front face such that the ceramic coating is in direct
thermal contact with the heat sink. A plurality of discrete thick film
conductive circuit pads are positioned on the electrically insulating,
thermally conductive ceramic coating and a thick film resistive layer is
positioned over portions of the conductive circuit pads such that the pads
are electrically connected in series through the thick film resistive
layer. The variable power resistor also includes a moveable contactor
capable of contacting the circuit pads in order to vary the resistance of
the resistor and means for electrically connecting the resistor to an
electrical circuit.
Another aspect of the invention includes a method of making a variable
power resistor, comprising plasma spraying an electrically insulating,
thermally conductive ceramic coating on a front face of a heat sink
followed by screen printing a plurality of discrete thick film conductive
circuit pads onto the electrically insulating, thermally conductive
ceramic coating. A thick film resistive layer is then screen printed over
portions of the conductive circuit pads such that the circuit pads are
electrically connected in series through the thick film resistive layer. A
moveable contactor capable of contacting the circuit pads in order to vary
the resistance of the resistor is also installed. The foregoing and other
features and advantages of the present invention will become more apparent
from the following description and accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a perspective view of the variable power resistor of the present
invention.
FIG. 2 is an elevation view of the variable power resistor of the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
The variable power resistor of the present invention is similar to a
conventional thick film power resistor, but dispenses with the separate
ceramic plate and thermally conductive grease or adhesive. In their place,
the present invention has an electrically insulating, thermally conductive
ceramic coating which is bonded directly onto the heat sink, making the
heat sink an integral part of the resistor. In addition, a new cermet
material is used to make the the resistive circuit compatible with the
integral heat sink.
As shown in FIG. 1, the heat sink 2 has a front face 3, which may be a flat
surface, and a back face 4. In addition, the heat sink 2 may have a hole 5
passing from the front face 3 to the back face 4 to facilitate the
attachment of the moveable contactor 6, shown in FIG. 2. The heat sink 2
may be made of any material which has a high thermal conductivity,
preferably at least about 1.16 Watts per centimeter per degree Kelvin (W
cm.sup.-1 K.sup.-1) (65 Btu hr.sup.-1 ft.sup.-1 .degree.F.sup.-1) at
25.degree. C. (77.degree. F.). Suitable materials include aluminum,
copper, and zinc. The preferred material is aluminum. The heat sink may be
any shape and size suitable for a particular application. For example, the
heat sink may be rectangular or circular, as appropriate.
Preferably, the heat sink 2 will have fins 8 which extend from the back
face 4 of the heat sink 2 to increase the surface area available for heat
transfer. As is well known in the art, the design of the fins is critical
to the rate at which heat can be removed from the resistor. The optimum
fin design depends on the material used to construct the fins, the amount
of heat to be removed from the resistor, the conditions at which heat is
to be transferred, the amount of space available for fins, and other
considerations. The preferred design for the variable power resistor of
the present invention includes fins which extend from one edge of the heat
sink to the opposite edge and have a rectangular cross-section. The
dimensions and spacing of the fins are design parameters which depend on
the particular application for which the resistor is intended. For
example, in a resistor with an aluminum heat sink designed to dissipate
100 watts of power, the fins may be about 0.050 inches (in) thick, about
0.200 in high, and may be spaced about 0.12 in to about 0.14 in apart. A
person of ordinary skill in the art would be able to design other fins
which would also be suitable.
The electrically insulating, thermally conductive ceramic coating 10 is
bonded directly onto the front face of the heat sink 2 so that the ceramic
coating is in direct thermal contact with the heat sink. "Bonded directly"
means that the ceramic coating is bonded without the use of adhesives,
grease, or mechanical means. While the ceramic coating may be any ceramic
which can be bonded directly to the heat sink, the preferred ceramic is
alumina. The ceramic coating may be bonded directly to the heat sink 2
using conventional plasma spray techniques. For example, the coating may
be sprayed with a commercially available plasma spray gun using an
argon-hydrogen or nitrogen-hydrogen plasma under conditions recommended by
the gun's manufacturer. If alumina is used as the ceramic coating, a Metco
3MB or 7MB gun may be used to apply the coating. The coating serves as an
electrical insulator between the resistive circuit and the heat sink 2,
which may be electrically conductive. In order to minimize the thermal
resistance introduced by the ceramic coating, the coating should be as
thin as possible without introducing the possibility of an electric short.
A coating of about 0.002 in to about 0.010 in would be effective.
Preferably, the coating will be about 0.002 in to about 0.003 in thick.
The resistor circuit comprises a thick film conductive layer 12, which is
positioned on the ceramic coating, and a thick film resistive layer 14,
which is positioned over portions of the conductive layer 12. The
conductive layer 12 may be screen printed onto the ceramic coating 10
according to conventional thick film techniques to produce a plurality of
discrete conductive circuit pads 16. The number of pads will depend on the
particular application for which the resistor is designed. The pads may be
of any convenient shape and size and may be arranged in any convenient
pattern. For example, the pads may be arranged linearly or in an arc.
Preferably, the circuit pads 16 will have roughly rectangular bodies,
arrayed in an arc around hole 5, with thin rectangular leads extending
from the bodies towards the hole 5. At least one of the conductive circuit
pads may be substantially larger than the rest to provide an area of zero
resistance. This pad, shown as the zero resistance pad 18, may also serve
as the electrical contact for current entering the variable power
resistor.
The conductive circuit pads 16 and zero resistance pad 18 may be made from
a conductive cermet material which comprises a mixture of a metal, such as
palladium or silver, reinforcing particles, and a glass. Cermets used with
conventional thick film variable power resistors typically fire at
temperatures of about 850.degree. C. (1560.degree. F.). However, because
the heat sink is an integral part of the variable power resistor of the
present invention, the cermets used with the present invention must fire
at a temperature below the melting point of the material used to make the
heat sink. For example, if the heat sink is made from aluminum, the
cermets used for the conductive circuit pads must fire below about
660.degree. C. (1220.degree. F.). One group of conductive cermets which
are suitable for use with aluminum and which fire at about 550 C
(1020.degree. F.) may comprise about 10 weight percent (wt %) to about 70
wt % of a lead borosilicate glass matrix, about 15 wt % to about 20 wt %
zirconium spinel reinforcing particles, and up to about 90 wt %
coprecipitated palladium/silver conductive particles in an amount
effective to provide a resistance of up to 0.5 ohms/square. Suitable lead
borosilicate glasses are available as SG67 from PPG Corporation
(Pittsburgh, Pa.) and 2143 from Drakenfield Colors (Washington, Pa.).
Suitable zirconium reinforcing particles are available as TAM 51426 Double
Silicate from TAM Ceramics, Inc. (Niagra Falls, N.Y.). Suitable
palladium/silver conductive particles are available as A-4072 from
Engelhard Minerals & Chemical Corporation (Edison, N.J.). The conductive
elements should be about 10 microns to about 40 microns thick. Preferably,
the conductive elements will be about 25 microns thick after firing.
The conductive circuit pads 16 and zero resistance pad 18 are electrically
connected in series by the resistive layer 14 which may be screen printed
over the pads 16, 18 using conventional thick film techniques which are
well known in the art. The resistive layer 14 is a resistive cermet which
comprises a low temperature glass, reinforcing particles, and metals. Like
the conductive cermet used for the conductive elements, the resistive
cermet must fire below the melting point of the heat sink. For example,
the resistive cermet may comprise about 5 wt % to about 70 wt % of a lead
borosilicate glass matrix, about 15 wt % to about 20 wt % zirconium spinel
reinforcing particles, and more than about 5 wt % coprecipitated palladium
and silver conductive particles in an amount effective to provide a
resistance of greater than to 0.5 ohms/square. The resistance of the
resistive cermet can be changed by altering the amount of metal in it. The
resistive layer 14 should be about 10 microns to about 40 microns thick
and preferably, will be about 25 microns thick. The resistive layer 14 may
be a continuous layer which covers a portion of each conductive circuit
pad 16, including a portion of the zero resistance pad 18, to form a
continuous electrical connection between the pads. A portion of each pad
16, 18 must be left uncovered to permit the moveable contactor 6 to
complete an electrical circuit through the pads. The size of the uncovered
portion is unimportant, as long as there is sufficient room to make an
electrical connection of low resistance. When covered with the resistive
layer 14, each of the conductive circuit pads 16 forms a discrete
resistor. The resistance of each resistor is determined by the composition
of the resistive layer, the distance current must flow through the
resistive layer to move from one conductive circuit pad to the next, and
by the width of the resistor.
After all of the layers have been deposited on the heat sink, the resistive
cermet can be laser or abrasively trimmed according to techniques well
known in the art to obtain a desired resistance. The trimming operation
entails the removal of a portion of the resistive cermet. Trimming may be
required because the cermet may be too thick or the firing conditions may
have altered its resistive properties.
The moveable contactor 6, shown in FIG. 2, may be any device which permits
an electrical contact to be made between the zero resistance pad 18 and
any of the conductive circuit pads 16. If the circuit pads 16 are arranged
in an arc as shown, the moveable contactor may have a rotatable shaft 20
which fits through the hole 5 in the heat sink 2. An arm 22 may extend
from the shaft over the exposed surfaces of the conductive circuit pads 16
and zero resistance pad 18. The shaft 20 and arm 22 may be made of any
material, although preferably, at least the shaft will be made from a
nonconductive material. Depending on the application, it may be desirable
for the nonconductive material to be a high temperature material. A
contact 24, which may be electrically connected to means for conducting
current out of the variable power resistor 26, makes contact with a
selected conductive circuit pad 16 to complete an electrical circuit from
the zero resistance pad 18, through the resistive layer 14, and through
the selected pad 16. The contact 24 may be a common button-type contact
like those found in switches and may comprise nickel-silver,
silver-cadmium, silver-copper, or any other suitable material.
Moving the contactor 6 from one conductive circuit pad 16 to another
increases or decreases the resistance of the variable power resistor. The
resistance selected is determined by the distance the current must flow
through the resistive layer 14 to complete the circuit. The lowest
possible resistance can be selected by moving the contactor over the zero
resistance pad 18. A knob 28 may be attached to the shaft to make movement
of the contactor easier. Of course, if the conductive circuit pads 16 are
arranged linearly rather than in the arc as shown, the moveable contactor
should slide rather than rotate.
Variable power resistors of the present invention can be sized to be
compatible with a wide range of applications. For example, depending on
the size of the heat sink, a variable resistor of the present invention
may dissipate between about 1 watt (W) to about 1000 W. The total
resistance of the resistor may range from about 1 ohm to about 10,000
ohms. Preferably the total resistance will be between about 10 ohms to
about 100 ohms. The resistance across an individual conductive circuit pad
is a design consideration and may range from as low as about 0.5 ohms to
as high as the total resistance through the resistor. Typically, the total
resistance of the resistor will be divided into convenient increments. For
example, in a 10 ohm resistor, 10 conductive circuit pads may be provided
so that the resistance across each pad is 1 ohm. However, it may be
desirable to arrange the conductive circuit pads so that each conductive
circuit pad has a different resistance. This would be particularly
desirable when each succeeding circuit pad is required to dissipate less
power that the one before it as the contactor moves from zero resistance
to the maximum resistance. The variable power resistor of the present
invention is compatible with voltages of up to at least 240 volts,
although it may most frequently be used at the 12 volts common in
automobiles.
A variable power resistor of the present invention provides several
advantages over conventional thick film power resistors. The use of a
plasma-sprayed ceramic coating between the heat sink and resistive circuit
facilitates fabrication. The present invention does not have a separate
ceramic plate insulator which can break during fabrication and has no need
for a thermally conductive grease or adhesive. The fact that the entire
resistor is built on top of the heat sink makes assembly easier. In
addition the absence of a ceramic plate insulator and thermally conductive
grease or adhesive improves heat transfer between the resistive circuit
and the heat sink. The plasma-sprayed ceramic coating has a lower thermal
resistance than the ceramic plate used in conventional thick film power
resistors because it is about 0.020 in thinner than the ceramic plate.
Moreover, because the ceramic coating is plasma sprayed directly onto the
heat sink there is no thermally conductive grease or adhesive to reduce
heat transfer or to dry out over time.
While the present invention is generally directed to a variable power
resistor, the combination of a circuit built on a ceramic coating which is
bonded directly to a heat sink or other metal substrate has far broader
application. For example, other circuits, including hybrid thickfilm
circuits which incorporate surface mount devices, could easily be built on
top of the ceramic coating. The substrate could serve as a heat sink, a
structural element, or as a ground plane as the application dictated.
It should be understood that the invention is not limited to the particular
embodiment shown and described herein, but that various changes and
modification may be made without departing from the spirit and scope of
the claimed invention.
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